Cooling devices utilizing the thermoacoustic effect have been attracting attention in view of their high reliability and other advantages due to fewer moving parts in comparison with cooling devices using compressors, etc. In addition, recently, they have been receiving attention from an environmental perspective as cooling devices that permit waste heat utilization and don't use chlorofluorocarbon gases.
As a first conventional technology, there is a thermoacoustic refrigerator made up of a tube, in which inert gas is enclosed as a working fluid, a loudspeaker arranged at one end of the tube, and a stack provided in the vicinity of an end portion of the tube (see, for example, “Thermoacoustic refrigeration”, Refrigeration, June 1993, Vol. 64, No. 788, by Steven Garrett (Steven L. Garrett), and one other). When the loudspeaker oscillates with a frequency that excites a standing wave inside the tube, the working fluid oscillates back and forth between the plates forming the stack and the pressure associated with the standing wave changes, generating adiabatic compression and adiabatic expansion, as a result of which the thermoacoustic refrigerator is cooled. The problem, however, was that performing heat exchange through efficient conversion of a standing wave to heat inside a stack was not easy.
As a second conventional technology, there is a thermoacoustic refrigerator with two stacks, wherein a standing wave and a traveling wave are generated by spontaneous oscillations in one stack inside a looped tube and a cooling effect is obtained in another stack (see, for instance, “Patent Publication No. 3,015,786”). It is noted that it has taken thermoacoustic refrigerators based on spontaneous oscillation roughly two decades to achieve success (see, for instance, “The Power of Sound (The Power of Sound)” (United States) by Steven Garrett (Steven L. Garrett) and one other, American Scientist, 2000, Vol. 88, p. 523, FIG. 8). As can also be gleaned from this, refrigerators utilizing the thermoacoustic effect had serious defects in that not only was it difficult to generate a standing wave and a traveling wave by self-excitation, but a certain time until the start of generation was required as well. It has been thought that the reason for that is due to the fact that the two stacks sandwiched between two heat exchangers in the looped tube that constitutes the device have to be arranged precisely in certain prescribed positions in the looped tube and, at the same time, if the shape etc. of the looped tube does not meet the prescribed requirements, it will not self-oscillate, and the standing wave and traveling wave will not be efficiently converted to heat. In other words, the greatest problem was to determine the requirements for spontaneous oscillation and to create an oscillatable device that would meet the requirements. In addition, another problem was that the device increased in size because the length of the looped tube had to be increased to lower the frequency of oscillation as much as possible and raise the efficiency of the thermoacoustic effect and/or output. Not only was it difficult, as describe above, to generate a standing wave and a traveling wave by self-excitation, but the two problems, i.e. the need for a certain time until the start of generation and the increase in the size of the device, greatly inhibited industrial applicability and impeded practical introduction and widespread use.